PCGetOperators#

Gets the matrix associated with the linear system and possibly a different one associated with the preconditioner.

Synopsis#

#include "petscksp.h" 
PetscErrorCode PCGetOperators(PC pc, Mat *Amat, Mat *Pmat)

Not Collective, though parallel mats are returned if pc is parallel

Input Parameter#

pc - the preconditioner context

Output Parameters#

Amat - the matrix defining the linear system
Pmat - the matrix from which the preconditioner is constructed, usually the same as Amat.

Note#

Does not increase the reference count of the matrices, so you should not destroy them

Alternative usage: If the operators have NOT been set with KSPSetOperators()/PCSetOperators() then the operators are created in PC and returned to the user. In this case, if both operators mat and pmat are requested, two DIFFERENT operators will be returned. If only one is requested both operators in the PC will be the same (i.e. as if one had called KSPSetOperators()/PCSetOperators() with the same argument for both Mats). The user must set the sizes of the returned matrices and their type etc just as if the user created them with MatCreate(). For example,

         KSP/PCGetOperators(ksp/pc,&Amat,NULL); is equivalent to
           set size, type, etc of Amat

         MatCreate(comm,&mat);
         KSP/PCSetOperators(ksp/pc,Amat,Amat);
         PetscObjectDereference((PetscObject)mat);
           set size, type, etc of Amat

and

         KSP/PCGetOperators(ksp/pc,&Amat,&Pmat); is equivalent to
           set size, type, etc of Amat and Pmat

         MatCreate(comm,&Amat);
         MatCreate(comm,&Pmat);
         KSP/PCSetOperators(ksp/pc,Amat,Pmat);
         PetscObjectDereference((PetscObject)Amat);
         PetscObjectDereference((PetscObject)Pmat);
           set size, type, etc of Amat and Pmat

The rationale for this support is so that when creating a TS, SNES, or KSP the hierarchy of underlying objects (i.e. SNES, KSP, PC, Mat) and their lifespans can be completely managed by the top most level object (i.e. the TS, SNES, or KSP). Another way to look at this is when you create a SNES you do not NEED to create a KSP and attach it to the SNES object (the SNES object manages it for you). Similarly when you create a KSP you do not need to attach a PC to it (the KSP object manages the PC object for you). Thus, why should YOU have to create the Mat and attach it to the SNES/KSP/PC, when it can be created for you?